1. Field of the Invention
This invention relates to a glue for wood, and more particularly to a glue for wood, comprising a modified polyvinyl alcohol having a hydrophobic group.
2. Description of the Prior Art
Generally, glues for wood are compositions which are obtained by causing thermosetting condensation resins such as, for example, urea-formaldehyde resin, melamine-formaldehyde resin, and phenolic resin to incorporate therein extenders and fillers such as wheat flour, starch, wood flour, nutshell flour, talc and clay. When such a glue is spread on a veneer for plywood, water contained in the glue permeates into the veneer or passes into the ambient air so that the glue starts drying up and setting before it undergoes thermal compression. Generally, this phenomenon is called dried out glueline, dry adhesion or presetting (hereinafter referred to collectively as "dryout phenomenon"). When the dryout phenomenon occurs during the production of plywood by union of veneers, the glue in use loses its adhesive strength. In an extreme case, the loss of the adhesive strength can be so serious as to induce layer delamination in the produced plywood. There is another possibility that not merely water but also the resin contained in the glue will permeate into the veneers. This phenomenon, called "glueline washout," entails loss of the adhesive force and layer delamination.
For the prevention of the phenomena of "dryout" and "glueline washout," measures such as (1) increasing the amount of glue applied, (2) increasing the water content of veneers to preclude the otherwise possible permeation, (3) causing the amounts of extender and filler contained in the glue to be increased beyond their usually accepted contents and thereby heightening the viscosity of the glue, (4) adding a cellulose derivative to the glue, and (5) adding a polyacrylic acid to the glue have been known to be effective. These measures have demerits of their own: The measure (1) is deficient in economy, the measure (2) suffers from poor capacity for temporary adhesion and, consequently, tends to entail development of contractions and warps in the plywood obtained by adhesion of veneers, the measure (3) results in a degradation of the adhesive force in water, the measure (4) impairs the adhesive force in water and the measure (5), though effective in improving the glue's resistance to the dryout phenomenon, fails to preclude the washout phenomenon and impairs the other physical properties.